Herbicides have been widely used to kill unwanted plants or weeds in crop fields. Glyphosate, also called Roundup®, is a common agricultural herbicide. This herbicide class functions by blocking the shikimate pathway in plants. The use of glyphosate resistant crop strains, also known as Roundup Ready® crops, in conjunction with glyphosate herbicide has increased crop yields and reduced costs for farmers. The use of glyphosate dramatically decreased the reliance on preemergent or residual herbicides as most production agriculture transformed to using postemergent applied glyphosate only. However, the emergence of glyphosate resistant weeds presents a growing problem for farmers. Weeds not killed by glyphosate necessitate the use of additional or larger quantities of herbicides, as well as a decrease in use of no-till farming methods, both of which increase the cost of farming.
Another class of herbicides is called Protoporphyrinogen oxidase (PPO) inhibitors. Herbicides in this class block the oxidation of the protoporphyrinogen within the chloroplast of a plant cell. Oxidation of protoporphyrinogen is critical for the production of protoporphyrin IX, a precursor molecule necessary for the metabolic synthesis of chlorophyll (a component of photosynthesis) and heme (an element of electron chain transport). Examples of PPO inhibitors include acifluorfen, carfentrazone, flumioxazin, flumiclorac, fluthiacet, fomesafen, lactofen, oxyfluorfen, pyraflufen-ethyl, saflufenacil and sulfentrazone. In practice, PPO-inhibitors are generally not used as postemergent herbicides across a wide-range of crops due to the potential for injury or killing the crops. They have limited uses as a foliar spray, primarily as a defoliant or desiccant. PPO-inhibitors are primarily applied preplant, or used weeks before crops are planted, to avoid potential phytotoxicty on emerging crop seeds from the residual remaining in the soil. This interval between application and planting allows residues to degrade for greater crop safety. However, preplant applications also reduce the longevity of weed control after the crops are planted because the herbicide starts to lose activity as crops are planted. This necessitates the need for additional weed control measures after planting. Some PPO-inhibitors contain use directions that require rainfall following the application and before crops are planted to mitigate potential crop phytotoxicity. Thus, the use of PPO-inhibitors as preemergence herbicide just before, at, or immediately after planting is relatively limited, or very crop specific, due to significant concern over crop injury. Resistance to PPO inhibitors has been documented, but not at the magnitude of glyphosate resistance, causing farmers and industry alike to caution the repeated application of PPOs as a single chemistry herbicide.
Fluridone, 1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridone, is a herbicide in the class of bleaching herbicides. Fluridone's mode of action inhibits the biosynthesis of carotenoid precursors. Carotenoid pigments protect chlorophyll from photodegredation. The inhibition of the synthesis of the precursors allows for the chlorophyll to become susceptible to degredation. Fluridone is commonly used in aquatic applications, and has also been used in agricultural settings, such as for controlling weeds in fields. Fluridone is sold under several trade names, including Brake™ (SePRO Corporation, Carmel, Ind.). Fluridone is a relatively broad spectrum herbicide that cannot be used in many crops due to the potential for phytotoxicity. However, cotton is one crop that is quite tolerant to fluridone.
In view of this background, need remains in the area of improved herbicidal methods and compositions useful for controlling weeds, particularly in an agricultural context.